Cavity resonator



Feb. 19, 1957 G, A, OLIVE 2,782,383

CAVITY RESONATOR Filed April 20, 1953 fama-1 /mwrae 650x905 A Oax/E fiar-1,

cAvirY REsoNAToR George Arthur Olive, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application April 20, 1953, Serial No. 349,869 3 Claims. (ci. S33-s3) tates Patent i cause each size resonator must be constructed with component parts especially designed for its own particular size of resonator.

It is an object of the present invention to provide'a novel cavity resonator of adjustable size.

A further object of the invention is to provide a cavity resonator which may be made any desired size, within limits, using standard component parts.

Another object of the invention is to provide a cavity resonator symmetrical about a central axis which is readily adjustable in radial dimensions.

A still further object of the invention is to provide a cavity resonator symmetrical about a central axis which may be constructed from standard component parts and which, nevertheless, may be pre-adjusted to any frequency within a desired frequency range.

Another object of the invention is to provide such a cavity resonator especially for use in conjunction with a standard type vacuum tube.

In accordance with the invention, the cavity resonator has two opposed flat metallic walls between which are held a plurality of vanes positioned radially about a predetermined axis and having those edges adjacent the axis preferably straight, parallel to the axis and normal to the opposed at walls. The vanes may be substantially planar, but other shapes may be employed. The spacing between any vane and that next to it is less than half the free-space wavelength, that is, less than cut-oif. Consequently, at their inner edges, that is the edges adjacent the axis, the vanes act as a virtual solid wall. Preferably these internal edges are positioned each at the same radius from the axis, and thus the resonator operates like a resonator having circul-ar symmetry. In one embodiment, the walls may be disassembled, the vanes moved to have their inner edges each at a new radius different from that in their former position, and the structure reassembled. The resonator then resonates at a different frequency, higher or lower according to whether the new radius and the cavity is smaller or larger. Fine tuning may also be provided. The flat metallic walls may have apertures to receive a centrally positioned discharge tube.

The foregoing and other objects, advantages, and novel features of the invention will be more fully apparent from the following description when read in connection with the accompanying drawing, in which like reference numerals refer to similar parts and in which:

Fig. l is a cross-sectional view of one embodiment of the invention in which the resonator with a tube inserted is used as a variable reactance;

Fig. 1a is a fragmentary top view of a portion of the tube of Fig. 1;

Fig. 2 is a bottom view of the embodiment of Fig. l;

Fig. 3 is a fragmentary top View of a modification of the embodiment of Fig. l, permitting easier adjustability of the resonator dimensions; and

Fig.`4 is a longitudinal crosssectional View of a modification of the lower portion of the arrangement of Fig. l.

Referring to Figs. l and 2, a plurality of planar rectangular vanes 1t) are held between a top plate 12 and a bottom plate 14. The vanes 10 may be received by grooves 16 in the plates into which they slidably t. The straight inner edges 18 of the vanes 10 are parallel to a predetermined central axis coaxial with the outer circular edge of the plates 12 and 14. The plates 12, 14 are held together and in firm mechanical and electrical contact with the vanes by circumferentially spaced screws 20 of which only two appear in Fig. l. The plates 12 and 14 are centrally apertured to be annular. At its internal edge, an annular dielectric spacer 22, which may be of mica, is inserted in top plate 12 in an annular depression formed to receive it. An annular coupling plate 24 is held against the mica by screws such as 26 insulated from top plate 12 by dielectric grommets 28. The coupling plate 24 has spring contact fingers 30 which hold and contact the external metallic cylinder 32 of a tube 34 inserted in the central aperture of top plate 12. The tube is of a familiar type. A convoluted cooling sheet of metal 36 (see Fig. la) within the cylinder 34 is welded at many places to a cup shaped anode 3S. Thus the fingers 30 make good electrical contact with the anode 3S through the convoluted metal sheet 36 and the outer cylinder 32. A screen grid 40 coaxial with and inside the cylindrical portion of anode cup 3S is made of a cage of wires each parallel to the axis, and indicated in dotted outline in Fig. 1. A control grid 42 surrounded by screen grid 40 and also coaxial with the anode 38 cup portion, but which may be Wound, is also indicated only in dotted outline. A coaxially positioned oxide-coated cylindrical cathode 44 surrounded by the screen grid is shown in dotted outline. The screen grid 40 cage at its bottom is extended into a solid frustro-conical skirt 46. A solid anode skirt 48 is extended from the bottom of anode 3S (the open end of the cup). A sealed glass cylinder 50 is sealed at a circumference of the anode skirt 48. This cylinder is extended to a press (not shown) of the tube. A metallic contacting screen grid cylinder 52 surrounds the press and is connected to the screen skirt 46, either throughout its circumference or by a suitable lead or leads sealed through the press. Heater leads are brought out to suitable contacts as at pin 54 sealed through the tube 34 press. A cathode lead is also brought out to a pin or pins as at 56, sealed through the press. The contr-ol grid lead is brought out to a pin 58 centrally positioned. The pins such as 54 and 56 are received by connectors such as 60. The central pin is received by a central connector 62. A solid cylindrical metallic shield 64, having its axis aligned with the axis of the tube, is connected through a metallic bushing 66 throughout the shield 64 periphery at one end of the shield to thebottom plate 14. A lead from the control grid connector 68 which receives the control grid pin 58 is brought out through an aperture in the shield 64 and insulated by a suitable grommet, as shown, for a purpose peculiar to the use to which this arrangement was put.

A dielectric web 70 including supports 69 and whi-ch may be pressed of a single piece of such ydielectric as polyethylene tetrafluoride (sold under the trade name of Teflon) holds the various connectors such as the central connector 68 and connectors 60. The non-centrally located pins have leads brought out through the iatented Feb. 19, 1957l armsl `ofthe web 7)` and aperturesin the bushing 66 to be readily available as at 72 for the application of desired direct current (D. C.) voltages. An lannular capacitive coupling pla-te 73 is spaced from a rim of the bottom plate 14 by an annular mica spacer 74. 4The coupling plate 75 contacts the screen grid connecting cylinder 52 with spring fingers 75. A cylindrical spacer 76 of low-loss ceramic dielectric holds the 'anodejannular coupling plate 24 in proper position and prevents accidental contact of screen coupling plate fingers 75 with the bottom plate 414 edges. The use of the dielectric cylindrical spacer 76 is known.

A pair of coupling loops 80 and 82 provide 'couplings to coaxial line sections @4 and 86 respectively. One line section S4 may be connected to a further line section short-circuited in any suitable known manner at a position of adjustable length from the coupling loop 80.

The other line section 86 may lead -to a suitable load` or be coupled to other circuit elements. The loops 86 and 84 may be adjusted by insertion to any desired extent `and valso by orientation in any plane as desired before the screws 20 'are tightened to hold the line sections in fixed position by blocks 87 which Ihold the line sections midway between the plates 12, 14. Thus the` screws when tightened cause both the vanes 10 and the line sections to be `held in place.

Assume a virtual solid metallic wall to lie on `a cylindrical surface defined lby the inner edges 18 of the vanes 1t). The remainder of the cavity resonator 78 of the invention is demarcated at the top from Ithe inner vane edges 18 by top plate 14 and annular anodecoupling plate 24, suiciently closely coupled that little, if any, energy is lost therebetween; by the anode connecting cylinder 32, along its outside surface from the. plane of the anode coupling plate 24, the lingers being so close that little energy escapes upward yat that point, and in any eventat the broad -contacts made by the fingers; by the bottom of the convoluted sheet 36, the convolutions being so closely yspaced that little energy is lost through them; then by the anode skirt 48,`iirst along the upper and outer surface of anode skirt 48 and then along the inner surface of this skirt 48; and then by the inner surface of the anode 38. At the bottom, from the inner vane edges 1 8; by the annular screen coupling plate 73 and lingers 75; by the screen grid cylinder 52 and skirt 46; and by -the screen grid 40 itself.

In this arrangement the'icoupling line 86 couples the cavity resonator to an oscillator tank circuit (indicatedby legend only). The reactance of the resonator is varied by varying the voltage by a signal on lthe control grid 49. When the tube is operated with suitable voltages applied to its elements, the signal on control grid 2 causes a variation of the electron stream density in the resonator lin the space between screen gridy 40 and anode 3S. This variation of the electron density varies the effective dielectric constant of the space or medium within the resonator #and therefore the reactance of the resonator. The variation rin reactance is reflected into ythe tank circuit to frequency modulate the oscillator.

Now consider the virtual cylindrical wall mentioned above. The virtual wall determines the radius of the resonator. The vanes 10 are arranged -normal to the top and bottom plates 12 and 14, In the region of fthe vanes, the resonator operates in a mode in which the electric vectors are normal to these plates. The vanes 1G, however, are spaced apart at less than a half wavelength as measured -in the dielectric between Athe plates. If the dielectric between the vanes is air, as the velocity of waves in -free space #and air is substantially the same,

Ithey are spaced at less than a half free space wavelength,

at the inner edges 18 and forso'meV distance outwardly.

This spacing between adjacent vanes therefore 'causes'v jacent Avanes 10A are Ynon-resonant atthe operating frequency. All of these spaces are open, except for the blocks 87 holding line section-s 84 and 86. It may also be observed ithat preferably the mode of resonance of the cavity resonator is such that the plates 12, 14 are spaced apart less than a half wavelength. Accordingly, waves at the resonant frequency cannot enter and propagate between any gadjacent pair of vanes 10. Instead, the waves are reflected at a virtual wall defined by the inner edges of vanes 1Q., The vanes 10 are not necessarily' planar, but should be separated by less than a half wavelength. However, a planar rectangular form for each of vanes 10 is desirable because the vanes then fit into grooves 16 and may more easily than otherwise be positioned as desired. Suppose, for example, that it is desired to decrease the frequency of resonance of the cavity resonator. The tube 34 is withdrawn. The sCreWsZlgare loosened. The vanes 10 are moved outwardly afterrremoval or loosening of the top plate 12,` so that theinner edges 18 lie on a new radius. The platesy 12,' 14 are then tightened to hold by the screws 20 the vanes in place and in contact at ltheir upper and lower edges respectively with plates 12 and 14. If the frequency is to be increased, the radius of inner edges 1S may be made smaller. Preferably, the inner edges are all lat the same radii, but each may be independently adjusted.L The grooves 16 keep the plates spaced at equal angular distances' around the axis, and assist in accurate positioning. If the plates were otherwise than straight and planar at their upper edges, `then the grooves could not hold them in radial planes that include the axis and at the same time permit their radial movement.

In one embodiment, the vanes were adjustable between extreme positions to provide a range of resonant frequencies from 650`mc./s. (megacycles per second) to 850 mc./s. The unloaded Q of the resonator was `in excess of 300 at selected frequencies throughout the range, including the extremes, and was over 390 at 850 Incl/s. This Q could be increased by silver plating all parts. In the embodiment lon which these measurements were made, most ofthe parts, including upper and lower pilates, were of brass.

Although the vanes may be held in place as shown, once the desired position for a particular resonant frequency range is known, the parts may be otherwise held in pla-ce, as by soldering `the vanes at their upper and lower edges tothe plates 12 and 14. Therefore, the parts can be` rnanufactured in quantity, and yet assembled for each particular resonator to be resonant at a different resonant frequency. Thus, resonators to be resonant at 850 mc./s vmay be made of the same parts as resonators to be'resonant at 650 mc./s., `or any frequency ingbetween. The parts need only be positioned differently in assembly. Moreover, the ltube 34 or one of the samesize, lits any of these resonators of different resonant frequency.

Fora ne tuning control, the short-circuited section of line` 84 may be employed. Fine tuning is accomplished by rotating the coupling loop Si?, by increasing or decreasing its insertion into the cavity having its virtual wall defined by thefinner vane edges 1S, or by adjusting the effective short-circuit distance or length of line 84. The effect Vis toincrease. or decrease the coupling and quantity of la suitablereactance into the resonator to change by a small amount its resonant frequency. The particulariptuning means shown (the variable length line 84 and the coupling variable by orientation of the loop) is known. 1 Other tuning means to provide line tuning may be employed.; Thus, the resonator according to the invcntion'may be employed in conjunction with known tuning means. Y

Referring to Fig. 3, an alternative embodiment Vhas an annulardisk 88 lying in a plane normal to the predetermined 'axis' inthe radial planes of which lie thev vanes 14). The disk 88 may be mounted in any suitable manner to rotate about the axis, and has cam slots 90. Cam followers 92 attached to the vanes 10 ride in the slots 90. The vanes may be held in contact, as in the embodiment of Figs. 1 and 2, in such a manner that they are readily released.

In the modification of Fig. 3, a change in the resonant frequency of the resonator is accomplished by releasing the plates so that the vanes 10 are loose enough to slip in the grooves 16, -turning the cam plate 88 to move all the vanes 10 in or out together, and again tightening the upper and lower plates 12 and 14 to hold the vanes 10 in their new position in contact with the upper and lower plates 12, 14.

In Fig. 4, there is illustrated one modification of the arrangement of Fig. 1 so that the arrangement may act as an oscillator. The shield member 64 in Fig. l may serve as a conduit for cooling air through the web 70 and the convolutions of sheet 36. In Fig. 4, the member 64 also serves as the outer conductor of a coaxial line section 94. This line section 94 may be short-circuited for the radio frequency energy by first and second annular plates 96 and 98, the plate 96 having spring lingers to contact the outer conductor 64 and the plate 98 having spring lingers to contact an inner conductor 100 coaxial with outer conductor 64. The plates are separated by an annular mica washer 102, and the assembly held together by bolts or screws 104 inserted through insulating grommets. Thus, the radio frequency short-circuit arrangement provides D. C. insulation. Suitable D. C. grid bias can then be applied by a connection (not shown) to the inner conductor 100 of line 94. The shortcircuit may be adjusted by a handle 106 to slide to any desired position.

When the line section 94 of Fig. 4 is adjusted to the proper electrical length to be resonant at or near the same frequency as the cavity resonator 78, and proper D. C. voltages applied to the tube 34, the arrangement oscillates. Feedback is obtained by the capacity coupling between screen grid 40 and control grid 42. If this coupling is ineffective or inoperable, other known means of securing the desired feedback in proper phase may be employed. The output may be taken from the line section 86 to a load. The tine tuning line section 84 may be retained, if desired, for tine tuning purposes.

The arrangement of Fig. 4 may be used as an amplifier by coupling into .the shorted coaxial line section 94 from the source of the energy to be amplified with a coupling loop `or probe (not illustrated) inserted at a suitable point.

It is apparent that the invention discloses a novel cavity resonator, the resonant frequency of which may be readily adjustable over a desired frequency range, or which may be manufactured from parts standardized for its type to resonate at a selected frequency within a given range.

What is claimed is:

1. A cavity resonator comprising two relatively flat disc-like members spaced from and parallel to each other, and a plurality of planar and rectangular platelike metallic vanes positioned edgewise, spaced from each other, and arranged at substantially equal angular intervals about a predetermined axis normal to said members, one of the two major dimensions of each of said vanes extending radially from said axis, saidvanes being held between and in contact with said members, the distance between each pair of adjoining vanes, measured adjacent their inner edges, being less than a half-wavelength at the resonator resonant frequency, whereby the said inner edges define one virtual wall of said resonator completely surrounding said axis, the spaces between the outer edges of adjacent vanes, and also between the inner edges of adjacent vanes, being open.

2. A cavity resonator comprising two relatively fiat disc-like members spaced from and parallel to each other, a plurality of planar and rectangular plate-like metallic vanes positioned edgewise, spaced from each other, and arranged at substantially equal angular intervals about a predetermined axis normal to said members, one of the two major dimensions of each of said vanes extending radially from said axis, said vanes being held between and in contact with said members, the distance between each pair of adjoining vanes, measured adjacent their inner edges, being less than a half-wavelength at the resonator resonant frequency, whereby the said inner edges detine one virtual wall of said resonator completely surrounding said axis, the spaces between the outer edges of adjacent vanes, and also between the inner edges of adjacent vanes, being open; and means for adjusting radially the position of at least one of said vanes, thereby to adjust the position of its inner edge.

3. A cavity resonator comprising two relatively at disc-like members spaced from and parallel to each other, a plurality of planar and rectangular plate-like metallic vanes positioned edgewise, spaced from each other, and arranged at substantially equal angular intervals about a predetermined axis normal to said members, one of the two major dimensions of each of said vanes extending radially from said axis, said vanes being held between and in contact with said members, the distance between each pair of adjoining vanes, measured adjacent their inner edges, being less than a half-wavelength at the resonator resonant frequency, whereby the said inner edges define one virtual wall of said resonator completely surrounding said axis, the spaces between the outer edges of adjacent vanes, and also between the inner edges of adjacent vanes, being open; and means for simul taneously adjusting radially the positions of all of said vanes, thereby to adjust the positions of the inner edges of said vanes.

References Cited in the le of this patent UNITED STATES PATENTS 2,306,282 Samuel Dec. 22, 1942 2,549,499 McArthur Apr. 17, 1951 2,551,672 Harris et al. May 8, 1951 2,562,323 Martin July 31, 1951 2,603,749 Kock July 15, 1952 

